The invention relates to a process for the preparation of middle distillates from a mixture of carbon monoxide and hydrogen.
The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting said mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropsch hydrocarbon synthesis process. Catalysts frequently used for this purpose contain one or more metals of the iron group together with one or more promoters and a carrier material. The products that can be prepared by using these catalysts usually have a very broad molecular weight distribution and in addition to branched and unbranched paraffins, they often contain considerable quantities of olefins and oxygen-containing organic compounds. Often only a minor portion of the products obtained consists of middle distillates. Not only the yield of the gas oil obtained, but owing to the presence of the afore-mentioned olefins and oxygen-containing organic compounds, its cetane number is unsatisfactory, as well. Consequently the direct conversion of H.sub.2 /CO mixtures according to the Fischer-Tropsch process is rather an unattractive route for the preparation of middle distillates on a technical scale.
In the present patent application the term "middle distillates" is used to designate hydrocarbon mixtures whose boiling range corresponds substantially with that of the kerosene and gas oil fractions obtained in the conventional atmospheric distillation of crude mineral oil. The middle distillate range lies mainly between approximately 150.degree. and 360.degree. C., with the fractions boiling between about 200.degree. and 360.degree. C. usually being referred to as gas oil.
Discovery was recently made of a class of Fischer-Tropsch catalysts which have the property of yielding a product in which only very small quantities of olefins and oxygen-containing organic compounds occur and which consists almost entirely of unbranched paraffins, which paraffins boil to a considerable extent above the middle distillate range. Owing to the high normal paraffins/isoparaffins ratio and the low contents of olefins and oxygen-containing organic compounds of this product, the gas oil present therein has a very high cetane number. It has been found that the high-boiling part of this product can be converted in high yield into middle distillates by hydrocracking. The feed chosen to be hydrocracked is at least the part of the product whose initial boiling point lies above the final boiling point of the heaviest middle distillate desired as final product. The hydrocracking, which is characterized by a very low hydrogen consumption, yields a product in which, owing to the high normal paraffins/isoparaffins ratio, the gas oil has a very high cetane number. The cetane number is one of the most important quality criteria for a gas oil when it is to be used as fuel for diesel engines. The gas oils used for this purpose are generally composed by mixing gas oils having a high cetane number with gas oils having a low cetane number. In view of the ample availability of gas oils having a low cetane number--such as cycle oils obtained as by-product in catalytic cracking--and the limited availability of gas oils having a high cetane number, there is an urgent need for the latter gas oils. According as a gas oil has a higher cetane number it will be a more valuable mixing component for the preparation of diesel fuels, since such a gas oil enables larger amounts of inferior gas oil to be taken up in the mixture and nevertheless enables the cetane number required in actual practice to be attained. In view of the fact that the above-mentioned two-step process offers the opportunity of preparing gas oils having a cetane number higher than 70, while the gas oils that are used as diesel fuel should have a cetane number of 40-50, it will be clear that the two-step process is excellently suitable for the preparation of valuable mixing components for diesel fuels.
The Fischer-Tropsch catalysts used in the first step of the two-step process contain silica, alumina or silica-alumina as carrier material, and cobalt together with zirconium, titanium and/or chromium as catalytically active metals in such quantities that the catalysts contain 3-60 pbw cobalt and 0.1-100 pbw zirconium, titanium and/or chromium per 100 pbw carrier material. The catalysts are prepared by deposition of the appropriate metals onto the carrier material by kneading and/or impregnation. For further information on the preparation of these catalysts by kneading and/or impregnation reference may be made to Netherlands patent application No. 8301922, recently filed in the name of the Applicant, in which there is also given a description of the above-mentioned two-step process for the preparation of middle distillates from H.sub.2 /CO mixtures.
Until recently, the two-step process was carried out as follows. The H.sub.2 /CO mixture used as feed was contacted in the first step at a pressure of 20-30 bar with the cobalt catalyst. Subsequently, the reaction product was separated at atmospheric pressure into two fractions, viz. a C.sub.5.sup.+ fraction and a fraction comprising the remaining reaction components, viz. C.sub.4.sup.- hydrocarbons, water, carbon dioxide and unconverted carbon monoxide and hydrogen. Finally the C.sub.5.sup.+ fraction, together with added hydrogen, was contacted in the second step at a pressure of about 130 bar with a catalyst containing one or more noble metals of Group VIII supported on a carrier. As regards this embodiment of the two-step process the following may be observed.
Although in the preparation of middle distillates according to the two-step process the part of the product of the first step whose initial boiling point lies above the final boiling point of the heaviest middle distillate desired as end product will suffice as feed for the second step, thus far the total C.sub.5.sup.+ fraction of the product of the first step was used for the purpose, since under the influence of the catalytic hydrotreatment the quality of the gasoline, influence of the catalytic hydrotreatment the quality of the gasoline, kerosene and gas oil fractions therein had been found to improve.
The high pressure used in the second step was thus far thought to be necessary on account of disappointing results obtained when carrying out the second step at a lower pressure. This may be seen from the following experimental results obtained in working up the product prepared by Experiment 13 of Netherlands patent application No. 8301922, also filed as U.S. counterpart application, Ser. No. 594618, filed Mar. 29, 1984, now U.S. Pat. No. 4,522,939, issued June 11, 1985 which is incorporated herein by reference. Contacting the C.sub.5.sup.+ fraction of this product together with hydrogen at a temperature of 300.degree. C. and a pressure of 130 bar with the Pt/SiO.sub.2 --Al.sub.2 O.sub.3 catalyst used in Experiment 20 of said patent application led to a product whose 200.degree.-360.degree. C. fraction had a high normal paraffins/isoparaffins ratio. A repeat of this experiment at a pressure of 20 bar and otherwise similar conditions produced a decrease in the yield of 200.degree.-360.degree. C. fraction as well as quite a sharp fall in the normal paraffins/isoparaffins ratio of this fraction. The latter phenomenon in particular is highly undesirable in view of the accompanying decrease in cetane number. In order to raise the yield of 200.degree.-360.degree. C. fraction, Experiment 20 was repeated at 20 bar, but using a lower space velocity. It is true that in this way there could be achieved a yield of 200.degree.-360.degree. C. fraction corresponding with that obtained in the experiment using 130 bar, but it led to yet a further decrease in the normal paraffins/isoparaffins ratio of the fraction.
As remarked hereinbefore, the two-step process was thus far carried out by separation at atmospheric pressure of the C.sub.5.sup.+ fraction from the product of the first step prepared at a pressure of 20-30 bar, and processing of said fraction together with hydrogen in the second step a about 130 bar. The assumption that by the choice of the H.sub.2 /CO molar ratio of the feed for the first step and the reaction conditions of the first step the quantity of hydrogen present in the product of the second step can be so regulated that this product contains sufficient hydrogen to carry out the hydrocracking in the second step led to the question whether it might be possible to carry out the present two-step process in "series-flow", which would considerably bring down the cost involved in the process. As already known, carrying out a two-step process in "series-flow" involves using the entire reaction product of the first step--without components being removed therefrom or components being added thereto--as feed for the second step which is carried out at substantially the same pressure as the first step.
Although in view of the cost involved carrying out the present two-step process in "series-flow" is much to be preferred to the procedure adopted thus far, there are two aspects which raise considerable doubt as to its practical possibilities. The first is the pressure. When the process is carried out in "series-flow", the second step should be carried out substantially at the same low pressure as the first step. As seen from experiments conducted earlier, a reduction of the pressure in the second step results in a very severe drop of the normal paraffins/isoparaffins ratio of the gas oil. The second aspect concerns the composition of the feed for the second step. When the process is carried out in "series-flow", the total reaction product of the first step is used as feed for the second step. In this connection it should be taken into account that generally no more than 1/3 of the reaction product of the first step consists of C.sub.5.sup.+ fraction while the remainder consists of water, C.sub.1 -C.sub.4 hydrocarbons, carbon dioxide and unconverted hydrogen and carbon monoxide. In consequence of the development of the Fischer-Tropsch reaction (CO+2H.sub.2 .fwdarw.--CH.sub.2 --+H.sub.2 O) the reaction product of the first step contains more water than hydrocarbons, expressed by weight. In view of the composition of said product--in particular the large amount of water present therein--it is very doubtful of course whether the noble metal catalyst used in the second step will still be able to bring about the desired conversion.
In spite of the expectations regarding the practical possibilities of the present two-step process in "series-flow", which, on account of the above observations, were bound to be negative, an experimental investigation in that direction was nevertheless carried out. Surprisingly, in this investigation it was not only found that carrying out the two-step process in "series-flow" leads to a yield similar to that obtained in the two-step process carried out in the conventional way, but also that the gas oil produced in "series-flow" has a much higher normal paraffins/isoparaffins ratio. An explanation of this surprising result can possibly be found in the fact that the composition of the feed for the second step is different. In addition to the C.sub.5.sup.+ fraction and hydrogen which are present in the feed for the second step when the two-step process is conducted in the conventional way, this feed now also contains C.sub.1 -C.sub.4 hydrocarbons, carbon monoxide, carbon dioxide and water. Apparently the presence of one or more of these components in the feed has so favorable an influence on the normal paraffins/isoparaffins ratio as not only to offset the afore-noted adverse effect of pressure reduction on the normal paraffins/isoparaffins ratio of the gas oil, but even to enhance this ratio considerably.